In general, an airbag apparatus for restraining an occupant by expanding and extending an airbag to a vehicle interior in collision or the like is mounted on a vehicle such as an automobile or the like. In such airbag apparatuses, various types of airbag apparatuses such as an airbag apparatus for use in a driver's seat internally installed in a steering wheel, a passenger airbag apparatus internally installed in an instrument panel, a side airbag apparatus internally installed in a side surface portion of a vehicle or a seat, a curtain airbag apparatus internally installed in an upper portion of a door, and so forth are developed and adopted. These airbag apparatuses are housed in a vehicle structure at a normal time such that the existence thereof cannot be recognized, and in an emergency such as a collision of the vehicle or the like, the airbag apparatuses are configured to expand and extend an airbag upon bursting through the vehicle structure. Hence, an airbag releasing portion of the airbag apparatus has to be configured to be able to smoothly and rapidly release the airbag in an emergency, whereas the same has to be configured to be covered so as to be assimilated to a vehicle interior component at a normal time.
Currently, FIGS. 4(A) and 4(B) are views illustrating a passenger airbag apparatus in which FIG. 4(A) is a cross-sectional view and FIG. 4(B) is an enlarged view of B-portion in FIG. 4(A). The passenger airbag apparatus is internally installed at a vehicle body side of an instrument panel 41 disposed at a front face of a passenger seat. Further, as illustrated in FIG. 4(A), the passenger airbag apparatus is provided with an airbag (not shown) that is folded back in the normal time and is expanded and extended in the emergency, an inflator 42 for supplying a gas into the airbag, a retainer 43 for housing the airbag and the inflator 42, an outer panel 44 mounted in the instrument panel 41, and an inner case 45 that is connected to an inside of the outer panel 44 and that latches the retainer 43. Incidentally, the retainer 43 is coupled with a fixing member 47 via a joining member 46 and fixed thereto.
In the aforementioned instrument panel 41, an opening portion 41a is formed so as to be able to release the airbag. In the opening portion 41a, the outer panel 44 is mounted using a clip 44a or the like and the surface thereof is processed so as to be assimilated to the instrument panel 41. Further, in an inside of the outer panel 44, a plurality of notches 44b for allowing the outer panel 44 to easily crack at a time when the airbag is expanded is formed. By means of the notches 44b, the outer panel 44 is sectionalized into a fixing portion 44c and a door portion 44d. 
The aforementioned inner case 45 is composed of an inner panel 45a connected to an inside of the outer panel 44, and side walls 45b that form a space for housing the retainer 43. In addition, a plurality of latching holes 45c is formed in the side walls 45b, and hooks 43a connected to the retainer 43 are configured to be latched with the latching holes 45c. Further, the inner panel 45a is divided into two parts at a center portion and each of the inner panels 45a is composed of a supporting portion 451 connected to the fixing portion 44c of the outer panel 44, a flap portion 452 connected to the door portion 44d, and a hinge portion 453 formed between the supporting portion 451 and the flap portion 452. The hinge portion 453 is substantially U-shaped as illustrated in FIG. 4(B), and normally, thicknesses of t1 through t4 are uniformly formed.
In the above-described passenger airbag apparatus, the gas is supplied into the airbag (not shown) by operating the inflator 42 in an emergency such as a collision of the vehicle or the like. The airbag is expanded in the retainer 3 and the inner case 45, and presses the inner panel 45a and the outer panel 44. The outer panel 44 is divided at the notch 44b between the door portions 44d and releases the airbag to the vehicle interior. As an expanding and extending operation of the airbag proceeds, the door portion 44d of the outer panel 44 is rotated toward an instrument panel 41 side, and is further divided at the notch 44b between the fixing portion 44c and the door portion 44d. Currently, since the fixing portion 44c and the door portion 44d of the outer panel 44 are respectively connected to the inner panels 45a, there is no possibility that the door portion 44d is broken up even when the outer panel 44 is divided at the notch 44b. Further, the door portions 44d are further rotated toward the instrument panel 41 side and are opened in a double door-like manner by means of the operation of a hinge portion 48 of the inner panel 45a. 
Here, the inner case 45 is formed of resin in general, and is susceptible to a surrounding temperature and is hardened or softened. Accordingly, a devisal is necessary for forming a shape of the hinge portion 453 so as to appropriately open the outer panel 44 in the double door-like manner. For example, a hinge portion described in Japanese Unexamined Patent Application Publication No. 2006-96267, is formed so as to expand out in a direction being spaced apart from an outer panel, and to be gradually thin-walled from a fixing portion side to a door-reinforcing portion side (refer to FIG. 4 in Japanese Unexamined Patent Application Publication No. 2006-96267). Further, a hinge portion described in PCT Japanese Translation Patent Publication No. 2003-137057, is formed such that a top of a bent portion has a thinner wall than the other portion of a curved hinge portion (refer to FIG. 4 in PCT Japanese Translation Patent Publication No. 2003-137057).
However, since the fixing portion side is formed to be entirely thick in the hinge portion described in Japanese Unexamined Patent Application Publication No. 2006-96267, a stretching up amount of the hinge portion at a time of opening the outer panel in the double door-like manner is small. Therefore, the lack of the stretching up amount has to be compensated by largely forming an entire hinge portion (namely, a diameter of a U-shaped portion). However, if the diameter of the U-shaped portion is enlarged, a non-welded portion between the outer panel and an inner panel increases, and an effect for reinforcing the outer panel is reduced. Furthermore, in the case that the U-shape is longitudinally formed so as to avoid such problems, a problem occurs in which a manufacture for the hinge portion becomes difficult and it becomes a factor of cost up.
Moreover, in a hinge structure described in PCT Japanese Translation Patent Publication No. 2003-137057, a portion (a top of a bent portion) that is easiest to be stretched at a time when the outer panel is opened in the double door-like manner is formed of a thin wall. Hence, a problem occurs in which an excessive stress is concentrated on the top of the bent portion.